1. Field of the Invention
The present disclosure relates to an oxide thin film transistor (TFT) and its fabrication method, and more particularly, to an oxide TFT having an amorphous zinc oxide semiconductor as an active layer, and its fabrication method.
2. Discussion of the Related Art
As consumer interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”), which substitute cathode ray tubes (CRTs), the conventional display devices, has increased. Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution, color display and picture quality, so they are commonly used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
The structure of a related art LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a related art LCD device.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel areas (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel areas (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
The foregoing LCD is light and has low power consumption, as such, the LCD receives much attention, but the LCD is a light receiving device, not a light emission device, having a technical limitation in brightness, a contrast ratio, a viewing angle, and the like. Thus, a new display device that can overcome such shortcomings is being actively developed.
An organic light emitting diode (OLED), one of new flat panel display devices, is self-emissive, having a good viewing angle and contrast ratio compared with the LCD, and because it does not require a backlight, it can be formed to be lighter and thinner. Also, the OLED is advantageous in terms of power consumption. Besides, the OLED can be driven with a low DC voltage and has a fast response speed, and in particular, the OLED is advantageous in terms of a fabrication cost.
Recently, research for an increase of a size of an OLED display device is actively ongoing, and in order to achieve such a large-scale OLED display device, development of a transistor that can secure constant current characteristics as a driving transistor of an OLED to ensure a stable operation and durability is required.
An amorphous silicon thin film transistor (TFT) used for the above-described LCD may be fabricated in a low temperature process, but has a very small mobility and fails to satisfy a constant current bias condition. Meanwhile, a polycrystalline silicon TFT has a high mobility and satisfying constant current bias condition but fails to secure uniform characteristics, making it difficult to have a large area and requiring a high temperature process.
Thus, an oxide TFT including an active layer formed with oxide semiconductor is being developed, but in this case, in an oxide TFT having a general bottom gate structure using oxide semiconductor, the oxide semiconductor has n-type characteristics, so the oxide TFT is fabricated to have a structure without an n+ layer, unlike the existing amorphous silicon TFT.
FIG. 2 is a sectional view sequentially showing the structure of a related art oxide TFT.
As shown in FIG. 2, the related art oxide TFT includes a gate electrode 21 formed on a substrate 10, a gate insulating layer 15a formed on the gate electrode 21, an active layer 24 made of oxide semiconductor and formed on the gate insulating layer 15a, source and drain electrodes 22 and 23 electrically connected with certain areas of the active layer 24, a protective layer 15b formed on the source and drain electrodes 22 and 23, and a pixel electrode 18 electrically connected with the drain electrode 23.
Unlike the existing amorphous silicon TFT, the related art oxide TFT is advantageous in that it is fabricated to have a structure without an n+ layer, so the process can be simplified.
Since the oxide TFT using oxide semiconductor as an active layer has excellent mobility characteristics, research into the oxide TFT is actively ongoing, and attempts of fabricating a back plane of a next-generation flat display by using the oxide TFT have continued, but research into the characteristics of the source and drain electrodes used in the oxide TFT have not been sufficiently conducted. In particular, as for the research into the source and drain electrodes, materials, contact characteristics, parasitic resistance, and the like, have not been sufficiently studied.
As mentioned above, the oxide TFT has an advantage in terms of the processing, namely, in that an n+ layer is not required to be formed, but such as advantage works as a disadvantage in terms of element characteristics. For example, in spite of the presence of a material having various work functions, since the source and drain electrodes are limited to use a material having low resistance, conductive material such as copper (Cu), molybdenum (Mo), a molybdenum alloy, or the like, is limitedly used as the material of the source and drain electrodes, and in case of the oxide TFT, a threshold voltage Vth is shifted to a negative voltage area due to the contact characteristics between the oxide semiconductor and the source and drain electrodes. As a result, it is difficult to form a driver integrated circuit (IC) on the substrate.